This thesis develops a 'response-dependent' account of the objectivity of ethical judgment. It uses the Kantian device of the Categorical Imperative (CI) as the critical reflective test for convergence in judgment that is genuinely objectivity-sustaining. After a discussion of the challenges to ethical objectivism, it is argued that Wittgenstein's consideration of rule-following provides a compelling case for rejecting attempts to ground objectivity in a manner that is external to normative practices. This removes one potent source of hostility to ethical objectivity, but it does not establish the truth of this position. Within the constraints of a broadly Wittgensteinian conception of objectivity, plausible accounts of ethical objectivity will be 'response-dependent'. The key issue facing such theories is whether they can provide a model of practical rationality that delivers suitably demanding, objectivity-sustaining standards of rational criticism. I examine two approaches to this issue. The first (suggested by John McDowell) takes Wittgenstein's rulefollowing considerations as implying a virtue-centred approach to ethical deliberation. On this view, virtuous agents' uncodifiable responses determine correct judgment. But the case for this theoretically modest model of objectivity has not been successfully made, and the approach has insufficient resources to yield genuine objectivity. Instead, appeal must be made to some more articulated account of practical rationality. The second, Kantian, model delivers this because the CI test provides a mechanism for objectivity-sustaining reflective criticism of agents' reasons for action, without importing any illicit codification of rationality. The CI can also be used for critical reflection on agents' appreciation of the ethical features of their situation. In this way the model can avoid the problem of empty formalism and offer a weak form of ethical cognitivism. This model also suggests a potential route to plausible approaches to some other problematic issues in ethical theory, such as akrasia and the categorical nature of moral reasons.